Congenitally quadricuspid aortic valves are rare and usually cause no valvular dysfunction. We studied 6 patients with quadricuspid aortic valves. One quadricuspid aortic valve was severely regurgitant, necessitating surgical replacement. The 5 remaining valves functioned normally and each of these 5 patients died from a non-cardiac condition. Other congenital cardiac abnormalities were present in 2 of the 6 patients: case 1 had a "ventricular septal defect" which had closed spontaneously and case 2 had a congenitally deficient posterior one half of the anterior mitral leaflet and no attached chordae tendineae to it. The accessory cusp in all 6 patients was smaller than any of the other 3 cusps which were similar in size. The accessory cusp was between the right and posterior cusps in 4 patients, between the left and posterior cusps in case 1, and between the right and left cusps in case 2. The right coronary artery was dominant in the 5 patients in whom coronary dominance was known.